Safe Torque Off (STO)

The STO function is the most common and basic drive-integrated safety function. It ensures that no torque-generating energy can continue to act upon a motor and prevents unintentional starting.


This function is a mechanism that prevents the drive from restarting unexpectedly, in accordance with EN 60204-1, Section 5.4. The Safe Torque Off function safely clears the pulses of the drive. The drive is reliably torque-free. This state is monitored internally in the drive.


STO has the immediate effect that the drive cannot supply any torque-generating energy. STO can be used wherever the drive will be brought to a standstill in a sufficiently short time by the load torque or friction or where coasting down of the drive is not relevant to safety. STO enables safe working when the protective door is open (restart interlock) and has a wide range of use in machines/ systems with moving axes, e.g. handling, conveyor technology.

Customer benefits

The advantage of the integrated STO safety function compared with standard safety technology using electromechanical switchgear is the elimination of separate components and the effort that would be required to wire and service them. Because of the rapid electronic switching times, the function has a shorter switching time than the electromechanical components in a conventional solution.